This invention relates to modified polyester films and specifically to polyester films which are useful as supports or as cover layers for photosensitive systems. More specifically, this invention relates to modified films which are predominantly polyethylene terephthalate and exhibit a high degree of permeability to water vapor without substantial loss in other physical properties.
The use of polyester films as supports and as cover layers for photosensitive systems is well known in the art. Most of these films are prepared from a polymer made by first reacting ethylene glycol and terephthalic acid or conducting ester interchange between ethylene glycol and dimethyl terephthalate, and then subjecting the reaction product to conventional polymerization conditions in the presence of a catalyst. This forms polyethylene terephthalate polymer which is subsequently cast into a film element. The resultant film element is then biaxially stretched, heat set and heat relaxed in order to improve the physical properties. Other procedures involved, such as the application of certain subbing layers, the coating of photographic emulsions and the like, the application of protective top-coats, etc., are all well known in the photographic art.
The need for films which exhibit good permeability to water vapor in photographic elements is disclosed in Land, U.S. Pat. No. 3,573,044. Polyester films, particularly polyethylene terephthalate films, would appear to be particularly useful for this purpose because of their dimensional stability and excellent clarity. However, polyester films are conventionally relatively impermeable to water vapor.
The present invention satisfies this need.